Rename OS X as macOS (#950)

Author: anfedorov

README.md

@@ -38,7 +38,7 @@ Getting Started with Slate
 
 You're going to need:
 
- - **Linux or OS X** — Windows may work, but is unsupported.
+ - **Linux or macOS** — Windows may work, but is unsupported.
  - **Ruby, version 2.3.1 or newer**
  - **Bundler** — If Ruby is already installed, but the `bundle` command doesn't work, just run `gem install bundler` in a terminal.